1. Field of the Invention
This invention relates to a flow meter utilizing a microbridge sensor, and more particularly directed to a zero point compensating circuit for use in such microbridge flow meter which is adapted to eliminate offsets occurring in outputs from the microbridge sensor.
2. Description of the Prior Art
A microbridge flow meter has a heater and upstream and downstream temperature sensors disposed on opposite sides of the heater provided on a bridge formed on a semiconductor substrate. The temperature sensors comprise two identical temperature sensing resistor grids R.sub.U and R.sub.D acting as thin film heat sensors, and the centrally located heater comprises a resistor grid acting as thin film heater. The temperature sensors R.sub.U and R.sub.D may be fabricated of any suitable, stable metal or alloy film, for example, permalloy. The sensor and heater grids are encapsulated in a thin film of dielectric, preferably made of silicon nitride, Si.sub.3 N.sub.4, to form thin film members. The temperature sensors R.sub.U and R.sub.D are comprised in a bridge circuit as shown in FIG. 1 to derive a bridge output V.sub.B.
In the microbridge flow meter thus constructed, a temperature provided by the heater and given to and sensed by the upstream temperature sensor differs from a temperature provided by the heater and given to and sensed by the downstream temperature sensor dependent upon flow rates of a fluid of interest. Thus, a flow rate is measured based on an output which is derived from the difference between temperatures sensed by the upstream and downstream temperature sensors. An increase in the flow rate causes the temperature of the upstream temperature sensor to be decreased, whereby the resistance of the sensor material is also decreased. On the contrary, the temperature of the downstream temperature sensor is increased and accordingly the resistance of the sensor material is also increased.
Preferably, the upstream and downstream temperature sensors are completely identical so as to respond temperature changes in the same manner. However, variations in physical characteristic inevitably occur in the manufacturing process. Such variations may cause a so-called offset, that is, a discrepancy between outputs from the upstream and downstream temperature sensors when there is no flow. Generally, these variations are suppressed below one per cent level of accuracy, so that the offset caused thereby is negligeable in large flow measuring applications, however, can damage precision in measurement, depending upon applications, particularly in measuring small flow rates of a fluid of interest.
To remove such offset, there are conventionally the following two methods:
1. The flow of a fluid of interest is stopped for a predetermined period for measuring and storing an output in this non-flow period, i.e., the offset value which is then subtracted from each output value upon normally measuring; and PA1 2. A non-flow condition is virtually created without stopping the flow of a fluid of interest for measuring and storing the offset value which is then subtracted from each output value upon normally measuring. PA1 a first switch means coupled to said heater means for turning on and off said heater means; and PA1 a current supply means coupled to said upstream and downstream thermal sensor means for selectively supplying said upstream and downstream thermal sensor means with a first current when said first switching means is closed and a second current larger than said first current when said first switching means is opened such that said upstream and downstream thermal sensor means reach a temperature to which they are heated when said heater is on in no flow condition.
The method 1 ensures to obtain the precise offset value, however, stoppage of flow of a fluid of interest, required by the method 1, is not always permitted.
The method 2, i.e., a virtual non-flow condition may be realized, for example, by turning off a power supply to the heater without stopping the flow.
It will be therefore noted that the method 2 is more practical than the method 1 for removing the offset.
Now, the method 2 will be hereinafter explained in more detail.
First, the resistance values of the upstream and downstream temperature sensor R.sub.U, R.sub.D at a reference temperature are designated R.sub.UO and R.sub.DO, respectively, and thermal coefficient of resistances (TCR) of the sensors R.sub.U and R.sub.D, .alpha.U and .alpha.D, respectively. Assuming that a temperature of the upstream and downstream temperature sensors R.sub.U, R.sub.D when the heater is off (in virtual non-flow condition) is T1 and a temperature of the same when the heater is on (normal measuring condition) is T2, the resistance value R.sub.U1 of the sensor R.sub.U at heater off time, the resistance value R.sub.D1 of the sensor R.sub.D at heater off time, the resistance value R.sub.U2 of the sensor R.sub.U at heater on time, and the resistance value R.sub.D2 of the sensor R.sub.D are given by the following expressions (1)-(4), respectively. ##EQU1##
In the circuit diagram of FIG. 1, it resistors R1 and R2 have the same resistance value, the bridge output V.sub.B at heat-off time (V.sub.B1) and heat-on time (V.sub.B2) are given as follows: ##EQU2##
If .alpha.U=.alpha.D and R.sub.UO .about.R.sub.DO stand, bridge output V.sub.B2 is not zero when the flow rate is zero, thus occurring offset. This offset, however, can be removed by subtracting heat-off bridge output V.sub.B1 from heat-on bridge output V.sub.B2.
The expression (5) is transformed by substituting the expressions (1) and (2) for R.sub.U1 and R.sub.D1 : ##EQU3##
Also, the expression (6) is transformed by substituting the expressions (3) and (4) for R.sub.U2 and R.sub.D2 : ##EQU4## Thus, ##EQU5##
It can be seen from the above expression that the offset caused by R.sub.UO .noteq.R.sub.DO in the bridge output V.sub.B2 is cancelled.
However, if .alpha.U=.alpha.D does not stand in the above method of cancelling the offset by subtracting the first bridge output V.sub.B1 from the second bridge output V.sub.B2, the resulting value V.sub.B2 -V.sub.B1 is not zero, and another offset occurs. This offset is smaller than the above.mentioned offset caused by R.sub.UO .noteq.R.sub.DO in the bridge output V.sub.B2, however, is not negligeable, particularly when small flow rates are to be measured.